We have two dogs (brother/sister) that are about 2.5 years old. They still play like that. It does sound like they are going to kill each other, but we know it is just playing.
If they were serious the hair on the back of thier necks would be up, they would not be doing thier "play stance" (front body down rear end in the air) and their tails would not be wagging.
Chances are they are just playing. I wouldn't worry about it to much unless it gets really rough.